This invention relates generally to removing contaminants such as PCBs from soil and more particularly to using an electrically-charged adsorbent electrode to enhance contaminant removal.
Hazardous waste sites are drawing increasing attention and concern, and the need to clean up such areas is becoming more and more apparent. Traditional cleanup operations are, however, very costly as well as technically difficult. They often involve the transportation of large volumes of contaminated materials to specially designated and/or designed dump areas. This transportation of large volumes of contaminated materials, often over relatively long distances, is usually one of the most significant cost factors in cleanup operations of this kind. Moreover, equally large volumes of uncontaminated materials such as soils are often brought from distant locations to refill the cleanup site.
Local incineration and in situ cleanup processes are the most common alternative cleanup methods. However, the fuel costs of incineration are quite large. Incineration may also lead to air pollution problems which are sometimes even more pernicious than the original soil contamination problem being addressed. In situ cleanup operations, which usually involve the injection of solvents into the ground and then the extraction of the resulting solvent/contaminant solution, require extensive drilling and sampling. Very large volumes of expensive solvents and/or water are required by such in situ methods. In situ cleanup methods are inconclusive because it is very difficult to assure the completeness of the cleanup. Portions of a contaminated area can thus be missed. Moreover, solvents can be lost in fissures in the earth and can escape to previously uncontaminated areas where the solvents themselves become contaminants. Solvent containment problems are often dealt with by isolating the contaminated area from adjacent uncontaminated areas using walls and screens of one kind or another. However, the cost of constructing these barriers often becomes prohibitive.
An alternative to in situ methods is ex situ methods in which the contaminated soil is excavated, treated, tested and then put back in the same location. Although the excavation cost is of concern, ex situ methods avoid the problems of extensive drilling and sampling, incomplete results and secondary contamination. Soil washing is one promising ex situ method for treating contaminated soils. The process typically consists of first screening the excavated soil and then separating fine particles (10-15 microns) from the soil using a hydrocyclone. The remaining soil fractions undergo attrition scrubbing to liberate the contaminants. A flotation cell is used to remove the liberated contaminants from the soil slurry. The contaminant is collected in the froth, while the cleansed soil is collected as the tails. Chemical additives are typically added to the flotation cell to produce a sturdy froth and facilitate collection of the contaminants. The residence time in the flotation cell has a direct impact on the cost of the overall process, wherein a shorter residence time reduces cost.
One known process used to reduce the residence time in flotation cells is the addition of surfactants which emulsify the contaminants. However, the surfactants generally cannot be separated from the soil for reuse. Thus, no recycling of the surfactants can occur. In addition, many surfactants can tend to produce strong froth which must be controlled to prevent spillage. Another step used to reduce residence time is the addition of adsorbent materials such as powdered or granular carbon. While such adsorbent materials are typically capable of being recycled, the separation of these adsorbent materials from the soil is a time consuming and costly process.
Accordingly, there is a need for a soil washing process which reduces the flotation cell residence time without using materials that are not easily recycled.